A simple and sensitive method has been developed for the routine assay of chloramphenicol and thiamphenicol utilizing an enzyme isolated from E. Coli. This method has been applied for the measurement of thiamphenicol and chloramphenicol in plasma. After oral administration of thiamphenicol (100 mg/ kg) only levels of 5-10 micrograms/ml of the drug was obtained in plasma. After intramuscular administration of chloramphenicol succinate in monkey drug disappeared very rapidly with a half life of 1-2 hrs., indicating that chloramphenicol should be administered more frequently in monkeys in order to maintain adequate antibiotic level.